M104 wiring loom - biodegradable insulation - remediation walkthrough (with photos)

Started by graham_v_neutral_bay, 07-04-2018 06:18 PM. 47 replies. 1,418 views.

07-04-2018 06:18 PM

Join Date: Mar 2018
Location: Neutral Bay NSW
Posts: 275
Car: 1990 W124 300CE-24

'92 W124 300E (M104). Wiring loom insulation is doing the biodegradable thing - the outer sheath has turned to crumbly powder. Documenting the remediation walkthrough with photos for anyone facing the same problem.

M104 wiring loom with degraded insulation exposed

Symptoms before remediation: random misfires under load, check engine light intermittent, AC clutch dropping out at random, RPM fluctuating at idle. Cause: the cloth-jacket portion of the M104 engine wiring loom (Mercedes-Benz part 124 540 27 32) is made of a 1990-1996 era biodegradable cotton compound that disintegrates after 30 years.

Remediation: full loom replacement is $1,840 from MB Australia. DIY wrap with new heat-shrink + butyl tape is $95 and works for another 20 years. I did the DIY route. Photos attached.

07-04-2018 08:42 PM

Vasilios_Billy

Lifetime Benzer

Join Date: Mar 2008
Location: Campbelltown NSW
Posts: 1,782
Club: M.A.L.A.K.A.S.
Car: 2020 AMG GT C

Graham - this is an important post.. the 1990-1996 M104 / M119 wiring looms are the worst single design liability of the W124 / W126 / R129 era. Mercedes-Benz quietly changed the insulation compound in late 1996 - if your car is post-October 1996 build date, the loom is fine for life. Pre that date, you will have this problem..

For anyone reading: the DIY wrap is acceptable as a long-term fix IF the underlying copper conductors are intact (no green corrosion at the splice points). If green corrosion is visible, the loom must be replaced because the conductor itself is degraded.

Test before deciding DIY vs full replacement: check resistance to ground on each circuit. If readings match the workshop manual values within 5%, the copper is intact and DIY wrap is fine.

08-04-2018 09:14 AM

Boonie_W108 Senior Member

Join Date: Feb 2014
Location: Marrickville NSW
Posts: 1,418
Club: M.A.L.A.K.A.S.
Car: '67 W108 250SE

GRAHAM U SAVED 1700 BUCKS WITH ELBOW GREASE. THATS GANGSTA. PHOTOS ARE GOLD.

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